In the present study, we co-injected cDNA encoding the CCK-B receptor into Xenopus oocytes as a model system to examine receptor antagonism at the single cell level. Our earlier work has shown that activating the CCK-B receptor causes marked desensitization of itself and also a cross desensitization of the mu opioid receptor. Hypothesizing that the agent of desensitization of the CCK-B receptor was a protein kinase, we therefore tested seven kinase inhibitors. Oocytes were incubated in these inhibitors for different times to determine if they had any influence over the recovery from desensitization of the CCK-B receptor or its downstream effectors. A total of 95 oocytes was tested. The inhibitors and their targets were staurosporine (nonspecific PKC inhibitor), aminopurvalanol (CDK1 and CDK2), SB 203580 (SAPK2a/p38 MAP kinase), SB202190 (p38 MAP kinase), PD 98059 (MEK), SP 600125 (JNK), and U0126 (MAPK cascade suppressor). Of these, U0126 and SB203580 clearly had no effect, but the other drugs seemed to block the desensitization (statistical analysis is still in progress). [unreadable] The second approach to examining the interactions between the mu opioid and CCK- B receptors uses native receptors expressed in isolated pyramidal neurons obtained from rat pups. Living slices of rat brain from the hippocampus region are dissociated by a small vibrating glass probe into neuronal cell bodies with some attached processes. Because no enzymes are used in the dissociation, fragments of presynaptic elements remain attached that announce themselves by spontaneous synaptic currents that we can record under whole cell patch clamp conditions. We have explored these acutely dissociated neurons extensively and have demonstrated that pre-synaptic fragments express both CCK-B and mu opioid receptors. Preliminary experiments show that the CCK-B agonist CCK8-S slows the rate of inhibitory post synaptic currents (IPSCs) and diminishes their amplitudes. The mu opioid receptor, activated by the agonist normorphine (20 micro molar), also slows the rate but seems to have no effect on amplitudes. These experiments demonstrate that both receptors can be present in the presynaptic fragments, and the next step is to determine if or how they interact.